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How come planes can endure more positive than negative g's? I.e. how come V-n diagrams are not symmetric?
In this question A380 reverse thrust on only half the engines some doubts were raised about the strength of structures in certain directions.
Specifically, Peter Kämpf states:

Aluminium and Carbonfiber are almost as good in compression as in
  tension, so the direction of the load is secondary. This is not true
  if buckling stiffness is not sufficient, but in case of wing in-plane
  bending, buckling is of no concern.

This made me wonder:
How come V-n diagrams are not symmetric?

From the image it can be seen that the structure at 150 knts can only cope with -1.8g, but +3.8g.
Where does this difference come from, is it coming from difference in buckling stifnesses in different directions?

The lines start at zero load factor because with no speed there is no lift. If you move further right, the upper line is determined by the maximum lift, while the lower line is determined by the minimum lift. Both have the same value only for symmetric airfoils - regular airfoils are optimized for creating lift in only one direction.
The upper and lower maximum load factor limits of the v-n-diagram are mostly determined by maximum gust loads in level flight, and in this condition they are represented by lines emanating from +1 g, such that the distance from +1 g in terms of positive and negative load factors should be the same. Add to this that your wings will create approximately 50% more force until they reach their maximum lift as compared to the minimum ( = maximum negative) lift (the details depend on the airfoil, of course), and then your load factor limits will be 1 + 1.5$\cdot$gust load factor in positive and 1 - gust load factor in negative direction.
For an example I have uploaded the polar of a supercritical airfoil which I calculated years ago with Mark Drela's ISES. It's the best I could find quickly - most polars only show the positive maximum lift, not the limit at the opposite end.

As you can see, the maximum $c_L$ is +1.3, while the minimum $c_L$ is only -0.8.
Let's look at a practical case: Utility category airplanes must be certified for +3.8 gs and -1 g. In level flight a positive vertical gust (updraft) might add +2.8 g. The same gust strength in negative direction (downdraft) will cause a load change of only -1.9 g (because the wing will stall earlier), which, when added to the regular 1 g at level flight will translate into a -0.9 g load factor. Close enough for limiting the negative load factor at -1 g.
At higher speed the gust will not stall the airfoil in either direction, and now we can simply assume that a gust will produce a positive load of 1 + gust load factor and a negative load of 1 - gust load factor. Let the gust load factor be 2.8, and your limits will be +3.8g and -1.8g, like in your example at 150 kts.
So far, I have looked at the rationale behind the load limits for certification. But also in real life there is a difference between positive and negative load factors: The wing spar has to carry the bending moment due to lift minus the wing weight. If it would need to carry the same aerodynamic loads in both directions (up and down), the wing weight would reduce the total positive load, but would make the total negative load greater. This is why the structure cannot endure the same load factors in both directions: You need to give the wing some margin to carry its own weight in addition to the aerodynamic loads.
Things look differently if you take the v-n-diagram of aerobatic category airplanes. Now you cannot readily assume that the plane will only fly straight, but you need to assume that it will be hit by a gust while flying upside down. Add to this that the symmetric airfoil will create the same lift in both directions before it stalls, and your envelope needs to be limited at ±6 g, making the envelope symmetric.

Hi Peter, I've changed the image to the better image you recommended, so perhaps your comments relating to my example are not correct anymore.

Do you also know the reason why the region of structural damage is different for positive and negative? The region of structural damage goes from +3.8 to +5.5 (where structural failure starts), so that's 1.7g difference. At the negative g range, the structural damage goes from -1.8 to -2.6, which is only 0.8g's difference.
Do you know what the reason is for this (large) difference?

@ROIMaison: The shaded area with structural damage is unusual already, and I guess the limits are artificial. Someone probably wanted to add that at 150% of limit load the structure is at its ultimate load and will fail. They just took the nominal g value and added 50% for the failure limit, without calculating exactly where the real limits are.

To put it simply, it is because the aircraft is designed to be stronger in the +G direction. It is not required to be as strong in the -G direction (since it will not experience high negative G loads during operations), so it is not designed that way. To do so would cause much unnecessary structural weight.

And the reason for this is probably that there are humans on board who tolerate a decrease of blood pressure in the brain better than an increase.

There are many engineering factors that determine g ratings.  The cumulative fatigue damage from dominant operations primarily determine g-ratings.  When landing, this contributes heavy fatigue positive g's.  When flying, the natural forces of gravity and turbulence (to include gusting winds) contribute to fatigue damage.  These stresses are felt throughout the control systems.  Therefore, the plane is engineered to be stronger in all its considerations to support and counter the greater fatigue concerns.
http://adg.stanford.edu/aa241/structures/structuraldesign.html
How come V-n diagrams are not symmetric?
V-n Diagrams tend to visually reflect some of the more dominant fatigue considerations.  Engineered considerations that influence g ratings also are reflected in the V-n Diagrams for similar reasons. Notice the sharp changes in shape in the following Vn diagram and its explanations:
http://adg.stanford.edu/aa241/structures/vn.html
So why not strengthen everything to have higher g limits? Cost and useful return on investment.  There is little practicality to have an aircraft stressed to +/- 100 g's if it has little useful load, is too heavy to fly, or costs so much very few customers can afford buying it and pay for its operation.
Balances in weight, cost, strength... are in constant flux throughout design to provide an overall useful and affordable aircraft.  Equal g ratings is not a priority, however, meeting regulatory minimums is a priority.
https://www.faa.gov/about/office_org/headquarters_offices/avs/offices/air/directorates_field/small_airplanes/media/CPS_Part_23.pdf
There are normal and abnormal conditions in which an airplane is stressed for negative g's.
Normal negative g's include: in-flight turbulence, recovery from a bounced landing, down drafts / wind shear ... and other engineered limits that are reflected in negative g ratings.
Abnormal negative g's include: walking on or otherwise loading the top of a wing, unrated aerobatics, excessive snow loading while parked, excessive pilot induced control moments causing negative g's, excessive road bumps while in transport ...
Conditions violating negative g design considerations include: improperly secured or excessively worn folding wing hardware, worn or warped wing gusset or related fastening hardware, fatigued or weak weldments, corrosion ...
G-ratings have to do with Damage Tolerance
As a point of reference.  Fatigue failures are usually more significant at gross weight, and therefore utility and ultimate g ratings (regulatory) are usually given at gross weight.
Strength of Materials is a particular area of science and engineering that characterizes properties of materials, including fatigue.  Computer simulations can run thousands of tests using variations in parameters to anticipate fatigue failures before they actually occur in flying aircraft.
An aircraft can be engineered to have greater negative g limits than positive g limits; however the cost for this non-useful engineered feature would likely be unacceptable to customers.  Lower useful weight to have the higher negative g support structure as an example.
How come planes can endure more positive than negative g's?
However, the FAA over many years of experience has set standards for minimum g ratings to help control accident outcomes.
As an example, see Part 25 AIRWORTHINESS STANDARDS: TRANSPORT CATEGORY AIRPLANES
Subpart C--Structure Fatigue Evaluation
Sec. 25.571
Damage-tolerance and fatigue evaluation of structure.
http://lessonslearned.faa.gov/Comet1/25.571.pdf
and an A/P's insights under the following debrief:
https://www.faasafety.gov/files/gslac/library/documents/2011/Jul/55585/FLYING%20LESSONS%20110623.pdf
In terms of engineering to meet regulatory requirements, the cumulative probability of fatigue failure resulting from negative g's is less than the cumulative probabilities of fatigue failure from positive g's.  The design limits reflect these failures through probability equations relating likeliness to occur and level of impact.
FAA SYSTEM SAFETY ANALYSIS AND ASSESSMENT (acceptable methods)
http://www.faa.gov/documentLibrary/media/Advisory_Circular/AC%2023.1309-1E.pdf
Methods of working probability equations:
http://people.qatar.tamu.edu/shehab.ahmed/ecen_459/Lec34-37.pdf
The information provided in these links are to show the effort involved is no small task for aircraft designers.  Many thousands of failure modes accumulate when every part, joint, human error, environmental, and every other factor potentially part of a failure is considered.
So when asking a designer what the effect would be on g-rating based on a specific failure mode of a part, is not a reasonable question to ask because of the extensive interactions with the total system to be considered.

@SMSvonderTann Personally, I think it's fine. It's not the brightest, to the point answer, but it's also not something I would want to see gone. Toni, perhaps you could improve the answer, and directly address the OPs points?


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Why are there no stall angle requirements outlined by regulations like CS-25?
When reading regulations like CS-25, it outlines specific requirements for stall speed such as
CS 25.103 Stall speed:

Source
The only mention of angle of attack is in relation to stick pushers.
Is there not something like an angle-of-attack margin that needs to be observed? Could one create a plane that has a very low stall angle of attack provided it meets the stall speed requirements?

I suspect that the AoA is not particularly relevant because it doesn't control how fast the plane lands. Absolute speed is correlated to safety through many factors, such as the energy involved in a crash, the amount of runway required, the stress on tires, etc... I can't think of how absolute AoA has an equivalent correlation to safety.

@KennSebesta, I understand what you're saying.
Still, the fact that stick pushers exist that get activated at a certain angle of attack means that there is some safety concern that is tied to angle of attack

There's no concept of x degrees is riskier than y degrees for a generic value of y. The AoA for a stick pusher to activate is dependent on the wing's airfoil. This is not the case for energy, and thus absolute airspeed.

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Why do most commercial aircraft have the Center of Gravity before the Aerodynamic Center?
I've looked at the other questions concerning this topic, such as:

How does an aircraft tailplane work?
Does static longitudinal stability require download on the tail?

But I'm still confused about why transport aircraft generally have the center of gravity before the aerodynamic center.
In this resource from Airbus, they write and show:

Source
Another example Airbus sketches the following picture

And writes:

As explained earlier, the more distant the CG and the CP, the bigger the pitch-down moment. Since for aircraft stability reasons the CP is always located behind the CG, a forward CG increases the distance between the CP and the CG. A CG position further forward than the most forward position of the operational envelope can affect the safety of the flight in many ways.

Why is the bold statement true? Couldn't you put the c.o.g. behind the aerodynamic center, and trim the aircraft with an upward lift force on the tail?
I understand that positioning the lift ahead of the c.o.g. is unstable (since the destabilizing moment increases), but can't this be fixed with an appropriate tail design?

Remember that you want it to be stable at multiple airspeeds, not just one. So the balances between pitching moment between wing and tail, and and the lift forces between wing and tail would need to track each other. Also, apparently, lifting tails are generally less efficient since the main wing is more efficient at producing lift forces and the balance of the numbers normally makes it more efficient to have the main wing produce a little more lift to counteract the downward tail.

It is this way so the aircraft will be stable.  Think about a dart -- the CG (the heavy brass weight) is far forward, while the aerodynamic center (the fins) are very far aft.
The dart can not trim -- so it is an incomplete example.  However, the fins provide great stability.
Get yourself a dart, a foam target board, and a safe place to throw.  Then do some experiments...  Start by throwing the dart normally.  Then throw it oriented 90-degrees to the direction of your toss.  Then 180 (backwards).  What happens?
Get a friend to do the tossing, but move so you can see the flight.  If the friend back up a bit - and throws somewhat slowly, you will be able to see the dynamic response of the dart through the air.
The dart is very stable -- because the aerodynamic center is behind the CG.

I wish I had more upvotes.  So often big words and math formulas get in the way of basic understanding very simple, observable, and demonstratable concepts.  This is something you can explain to a 5 year old.  (no insult to the OP intended...)

@MichaelHall: albeit being formally correct for a dart, this answer cannot be extend to an airplane since an airplane possesses not only a tail but also a wing and a fuselage that, by an aerodynamic point of view, complicate things quite a bit. For example the dart analogy cannot explain why a canard is stable too, even if "the tail" is "in front": according to this answer the Wright brothers couldn't have flown... I agree with you that "often big words and math formulas get in the way" but sometimes that's the only way to understand/explain things in a correct and general way.

@sophit, agree completely.  In this case the question was well thought out and deserved the details provided in the other excellent answers such as yours.  Other times i wonder sometimes if people over answer based on my perception of the asker's base knowledge...

@sophit The question has been modified since I answered it.  The question was not about all aircraft or canard aircraft. The question is clearly about transport category aircraft -- all of which have been a wings forward, tail aft design (except the Concord & TU-144).  When engineers first look at static stability, we ignore the fuselage aerodynamically -- considering only the lifting surfaces.  I am usually quick to go to equations and rigorous theory -- I am certainly not shy from it.  However, this question 'Why is the CG in front of the AC' can be concisely answered 'For stability'.

I also ignored the fuselage in my answer  I definitely didn't want to criticise your answer, I just wanted to broaden the comment of @MichaelHall

I appreciate the answer, but I have the feeling the simplification to a dart removes the part which is confusing me.
In the case of a dart there is only a lifting surface and a center of gravity, so the required placement is clear. However, in case of an aircraft there are three 'elements', the lifting surface, the center of gravity and the tail. The way I see it, the addition of the tail should allow the center of gravity to be placed behind the wing (as it can provide a moment opposing the wing by providing a force upwards)

When you use the concept of the aerodynamic center, you replace two wings with a single one.  Think of the AC as a weighted average position of the wing.  So the dart is a reasonable example (though it can not trim).  When you place the wing in front of the CG, the aircraft becomes unstable.  We're monitoring the slope of the Moment vs. Angle curve -- dM/da.  When that is negative (CG in front of AC) it is stable.  When that is positive (CG behind AC), it is unstable.

I'm shocked - shocked! - to see this inaccuracy from Airbus. Or am I?
Of course the bold statement is utter nonsense, but is repeated over and over. Which doesn't make it true, but very hard to dispel.
Improvements in efficiency over the years have demonstrated that at least the engineers understand the matter correctly. But I'm afraid they have no time to constantly check on the nonsense their marketing apprentices publish.

Why do most commercial aircraft have the Center of Gravity before the Aerodynamic Center?

Not most, but all of them have their Center of Gravity ahead of the Aerodynamic Center. This is a certification requirement. However, with relaxed static stability, it is quite common for the center of pressure of the wing to be forward of the center of gravity, so that the tail is slightly positively loaded.

Couldn't you put the c.o.g. behind the aerodynamic center, and trim the aircraft with an upward lift force on the tail?

You can put the center of gravity (CG) behind the center of pressure (CP) of the wing. Only in landing configuration, when the CP of the wing is shifted backwards due to flap extension, will downforce on the tail be required. However, for natural static stability you need to have the CG ahead of the aerodynamic center at all times.

I understand that positioning the lift ahead of the c.o.g. is unstable

Actually, both lift and center of gravity should be at the same lengthwise station in order to enable straight flight. Stability is a consequence of the lift vector derivative over angle of attack.

Aerodynamic Center and Neutral Point are different things?  There is a a lot of confusion over that.

@JohnK Where did I say that? No, they should be the same. I thought I only mentioned the center of pressure (which indeed is a different thing).

I was just clarifying. You said CofG is behind the AC/NP except during landing.  Isn't that negative static margin? How do you achieve positive static stability with negative static margin? Don't you mean C of G behind the wing C of Pressure?

@JohnK Oops, I mixed up center of pressure (CP) and neutral point (NP). What is ahead of the center of gravity (CG) is the center of pressure of the wing (at least in Airbus airplanes with relaxed static margin). Of course the neutral point of the whole airplane is still aft of the CG. With the large shift in the CP with extended flaps, this changes and now the CP of the wing is also aft of the CG (which changes only a little when flaps are deflected). In steady flight in all phases the CG will coincide with the CP of the whole airplane.

@JohnK I should read the questions more carefully. I only focused on the bold line from the Airbus quote. Now I understand your doubts, and thank you for pointing out my mistake.

@PeterKämpf You mention "Not most, but all of them have their Center of Gravity ahead of the Aerodynamic Center. This is a certification requirement.", why is this the case? You can have a center of gravity behind the AC, and rely on the tail to ensure CM/dAlpha is negative?

@ROIMaison I was referring to the AC of the whole airplane. Of course the tail makes the pitch moment gradient over Alfa negative but also shifts the AC of the whole airplane back behind the center of gravity (which is essentially the same).

As Peyer Kampf points out, Confusion between definition of terms and the diagram. The. ***OVERALL center of pressure or aerodynamic center of the entire airframe, must be aft of the CG, but the diagram does not depict that. It only depicts the AC of the main wing.  You are exactly correct that the AC of the main wing can be ahead of the CG if you add a horizontal tail structure far enough aft to move the total AC behind the CG. This is not common, (because if makes the dynamic stability, what happens as AOA changes, more complex and problematic), but is possible

I have experience flying flying free-flight model airplane hand-launched gliders with the CG typically located at 60% of the chord or even further back. This allows the planes to shoot up 200 feet or more at tremendous speed, then come out into a slow circling glide, all with no moveable control surfaces. It also causes the planes to crash a lot, not recover well from disturbances and be extremely sensitive to small warps. So it is possible to have just barely enough longitudinal stability with an aft CG and lifting tail, but at a loss of the kind of safety that human-carrying craft require. The more aft the CG, the more problems. Free-flight contest models carry this to a ridiculous degree.
Traditional contest free-flight models are aerodynamically optimized for the greatest possible speed difference between the climb and the glide. The fact that this leads to a lot of crashing has caused modern designs to incorporate control surfaces automatically actuated at the transition to glide. Contests are won by rapidly getting high enough to ride thermals at the same speed a hawk does.

Actually, the diagrams are sort of a "tongue in cheek" way of saying we don't fly staticly stable planes because we want to save fuel costs

"your winnings, sir"

Over the life of the aircraft, flying staticly neutral but directionally stable can indeed result in considerable savings.

can it be done safely, even with swept wings?

Here's where it's important to realize that these huge airliners don't scale very well with smaller aircraft when it comes to stall recovery.
That's why it is so important to understand all sources of pitching moments, including large underslung fans that, by design, create a "pitchup" moment in cruise so not as much tail downforce needs to be applied to counteract wing moment.
Unfortunately, these gigantic fans, low and slow, can produce so much pitching moment at TOGA thrust that the tiny tail cannot keep up, especially if the CG is too far back.
So, it's a bit more complicated, but I might not mind as a passenger, as long as the builders of the aircraft aren't fooling themselves.

The reason for placing the CoG in front of the neutral point is the behaviour when the Angle of Attack increases: for passive stability, we want gravity to provide a nose-down moment: $dC_L/d\alpha$ must be < 0, as explained in this answer
An intuitive case occurs when the wing enters a stall: the lift vector decreases, and if the CoG is behind the CoL, the reduction of the lift vector causes a nose-up moment. This further stalls the aeroplane, which is not what we want.

But, arguably, gravity does not create a moment at all, right?  I mean, sure, in the other answer you linked to it does, but that's due to an arbitrary choice of station zero way out in front of everything else, right?  To purely work a torque balance problem, we are free to pick any point we want to resolve the torques around, but if there is any acceleration going on, then we must pick the CG to resolve everything around, unless we want to complicate the picture by introducing fictitious inertial forces ("centrifugal force").  Doesn't that pretty much sum it up?

Are you saying that you want gravity to create a nose-down moment about the Aerodynamic Center, perhaps for reasons alluded to in this answer to a related question https://aviation.stackexchange.com/a/50380/34686?  And if there is any acceleration tangent to the flight path, shouldn't you really say that you want "gravity + centrifugal force" to create a nose-down moment about the Aerodynamic Center?"  (I think... actually still thinking that last one over...)

@quietflyer the aeroplane was trimmed before losing lift. After the stall there is a change in moment and in vertical force due to the loss of lift, therefore it descends and rotates, and we want the rotation to be nose down. Indeed pick any point for the moment equilibrium - the tailplane is a convenient one for this case.

An airplane (or any other mechanical system) is stable when, after something has perturbed its attitude, it tends to go automatically back to its initial condition.
In particular, an airplane is longitudinally stable if an increase in the AoA (the perturbation) will create a nose-down pitching moment which makes the AoA decrease, thus restoring the initial condition (picture source):

What are the sources of pitching moment for an airplane? To simplify a bit the discussion we limit the analysis to a conventional airplane (i.e. an airplane with a well defined wing, fuselage and tailplane) and we omit, as is customary, the contributions coming from drag, engine(s) and  fuselage. We have then:

lift $L$ of the wing; lift acts in the aerodynamic center of the wing which lies at a distance $x_{ac}$ from the nose; the aerodynamic center is at 25% of the chord at subsonic speeds and 50% at supersonic speeds;
aerodynamic pitching moment $M_{wing}$ of the wing; it is normally negative and more or less constant with the AoA;
lift $L_{tail}$ of the horizontal tailplane, which acts at a distance $x_{ac_{tail}}$ from the nose.

The following figure (source, modified by me) shows schematically these loads:

Now we only need a point in respect to which to calculate the total pitching moment $M$: we use the CG of the airplane since it gives some useful results. Then summing up all these terms in respect to the CG we get:
$M=L(x_{cg}-x_{ac})+M_{wing}+L_{tail}(x_{cg}-x_{ac_{tail}})$
If we equal this equation to zero, we get the conditions needed to equilibrate i.e. trim our airplane. Anyway to get its stability we need to go a step further and apply the previous definition: the airplane is stable if any perturbation that changes the AoA creates a pitching moment $M$ that brings the AoA back to its initial trimmed value. This means that the aircraft is stable if a positive change of AoA creates a negative change of moment $M$, i.e. if their ratio is negative:
$\frac{∆M}{∆\alpha}<0$
where $∆$ means "variation". Applying this definition to the previous expression we get:
$\frac{∆C_M}{∆\alpha}=\frac{∆C_L}{∆\alpha}(x_{cg}-x_{ac})+\frac{∆C_{L_{tail}}}{∆\alpha}(x_{cg}-x_{ac_{tail}})\frac{V_{tail}²S_{tail}∆\alpha_{tail}}{V²S∆\alpha}$
which is the "stability equation" of the airplane. A couple of comments before going further:

as usual in the aerospace world, moments and forces have been adimensionalised via $½\rho V²S$ and written in coefficient form;
$M_{wing}$ disappeared; as said, wing's pitching moment is basically constant in respect to AoA and therefore its variation is simply zero;
the very last term just translate the fact that the aerodynamic characteristics of the horizontal tailplane must obviously be given in respect to its $(½V_{tail}²S_{tail}∆\alpha_{tail})$ while the adimensionalisation has been done in respect to the wing's $(½V²S∆\alpha)$; worthy of remark is the fact that both the speed $V_{tail}²$ and the AoA $\alpha_{tail}$ seen by the horizontal tailplane are different than the ones of the wing; this is due to the downwash of the wing impinging on the tailplane which reduces both.

Now we only have to set $\frac{∆C_M}{∆\alpha}<0$ to get the aeromechanical settings needed to have a stable airplane. Since we have the sum of two terms, in order for this equation to be negative at least one of these terms must be negative and more negative than the other.
We start from the tailplane:

$\frac{∆C_{L_{tail}}}{∆\alpha} \rightarrow$ this is the slope of the lift curve which is always positive (bigger $\alpha$, bigger $C_l$);
$\frac{V_{tail}²S_{tail}∆\alpha_{tail}}{V²S∆\alpha}\rightarrow$ this is the ratio of geometric entities and is also positive;
$(x_{cg}-x_{ac_{tail}})\rightarrow$ if the horizontal stabiliser is located on the tail, then this term is negative; bingo! The horizontal tailplane gives a negative contribution to stability and therefore it is always stabilising.

Now the wing:

$\frac{∆C_L}{∆\alpha}\rightarrow$ the slope of the lift curve is positive;
$(x_{cg}-x_{ac})\rightarrow$ this term is positive or negative according to the relative position of CG and AC; if the CG is in front of the AC then this term is negative (i.e. stabilising) and viceversa.

What do we choose now? CG first or AC first?

But I'm still confused about why transport aircraft generally have the center of gravity before the aerodynamic center.

If we want to be conservative then we put the CG in front of the AC: should the tailplane lose some of its effectiveness, the airplane will remain stable. Plus, if the CG helps in stabilising the airplane then a smaller tailplane can be used giving less drag and weight.

Couldn't you put the c.o.g. behind the aerodynamic center, and trim the aircraft with an upward lift force on the tail?

Yes definitely, we can we put the CG between the wing and the tailplane but then $(x_{cg}-x_{ac})$ becomes positive (i.e.destabilising) and the tailplane needs to be bigger to compensate also for this instability due to the CG. Anyway this configuration with the CG between wing and tailplane entails that the horizontal tailplane produces a positive lift relieving a bit the job of the wing.

Don't forget those little ventrals in the back. They help too.

@RobertDiGiovanni: only talking about longitudinal stability here.

Though not quite as good as endplating a horizontal stabilizer, as AoA increases, you may find a pocket of higher pressure there.

It has nothing to do with stability in normal forward flight, and a slightly rear CG with up-lift on the tail will work just fine and be rather efficient.
The problem is recovery from upset like a stall or spin. With a rear CG the aircraft will tend to stay tail low and cannot reduce the AOA enough to recover the forward airpeed needed for control authority in any axis. And so you fall, fall down to the ground.
look at the top down planform of the aircraft and imagine the distribution of vertical drag on the surfaces of an airplane that you just drop at zero forward airspeed.(an absolute stall)
CG forward and the drag on the tail surface causes the nose to drop and the plane develops some relative forward airspeed. That is forward airspeed relative to the AOA, not necessarily horizontal over to the ground.
With an aft CG the tail drops first or equally and the plane can never develop airflow in the direction needed to effect pitch or yaw.(related to a flat spin)
The way around this CG efficiency-safety tradeoff is with a canard planform. Though the canard form has some other engineering tradeoffs. (Canard is not a term for a forward horizontal stabilizer. Canard is French for duck, because a mallard duck in flight similarly has a planform with very little tail aft of the main wings and a long neck out front.)

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How much fuel is burned to carry the trip fuel?
For rockets, there is a rule of thumb that says for each kilo of fuel put into orbit, 7 kilos is needed to get it there.
For long and ultra-long flights, it feels intuitive that some very significant proportion of the fuel, perhaps the majority, is needed to carry the fuel needed to complete the flight. In other words, imagine that the fuel was weightless and 50,000 litres were needed. But since fuel is not weightless, the requirement is actually 100,000 litres.
Of course there are a lot of variables and it's fairly easy to calculate, but is there a rough and ready rule of thumb?

Not an answer, the Flight Planning & Performance Manual includes a table for break even cost of tankering. E.g. for the B737-800. It varies with the duration of the trip, 3 to 5% per hour seems agreed. Indeed at 4%, 10 hours is much much more than 40%.

The performance charts mau be used to extrapolate the data.  Take the performance without any fuel on board and subtract it from the performance with fuel on board.

Have a look at Sustainable Energy without the Hot Air Appendix II C - Planes. It is a look at flight from an energy point of view and may give you an alternate perspective on some aspects.

For what it's worth, in the 1990s on 747-100/200 aircraft, we used 80% as the recovery rate of extra fuel beyond what dispatch planned for us on flights from KJFK into western Europe, flight time of 7 to 8 hours. In other words, if we loaded 5000 lbs extra, we would burn 1000 of that just to carry it.

While various operations will have various rules of thumb, I don't think that there is any one single rule of thumb, because the length of the flight (and probably, the type of aircraft) affects the result to much.
In a fairly short-haul 737, to get "another 1000 lbs" of fuel at landing, you just upload another 1000# into the tanks.  You'll burn less than 10 pounds of that for a quick flight, and so uploading the extra 1000# means arriving with about 990# more, which is too small a difference to worry about.  (Plus, you order fuel in 100 pound increments, so "load another 1007 lbs" would generate a puzzled look from the fueler.)
On the other hand, in a discussion with a 747 pilot who routinely flew from Minneapolis to Tokyo, when he wanted another 10,000# of fuel at arrival (that airplane uses bigger numbers than the smaller 737!), he had to upload almost 20,000#.  The 737 rule of thumb wouldn't work at all for him, as his rule of thumb was based on burn rates and timeframes that totally outside of the 737 operation.

a fascinating and excellent answer!

I'm not sure you have answered the question... how much fuel was burnt only to carry the fuel? You said that adding 1,000 lbs to what is needed for  the flight will lead to 10 lbs burnt for carrying 990 lbs to destination. But how much was burnt to carry the rest that was burnt along the flight?

You're right that the question was more about the total fuel cost to carry all fuel, while my answer was more about marginal cost to carry marginal fuel, and those aren't entirely the same. My bad not catching that difference on first read. Nevertheless, for the same reasons as mentioned, I think "rules" not "rule" of thumb is still correct, and the ballpark of 10# delta burn per 1000# delta weight will probably still apply to the 10k - 30k of fuel typically carried on a short-haul 737.

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What systems, in the cabin ceiling, when damaged could cause the masks to deploy?
I feel guilty when I chuckle at this incident.
Does anyone know what system(s) might be contained in the cabin ceiling which when struck by a high speed champagne cork could lead to deployment of the passenger oxygen masks?

According to the AV Herald report, it wasn't all the passenger oxygen masks, but rather a few masks for cabin crew, presumably those located over the jumpseats near the galley. The masks are generally covered by a drop-down panel, which opens either electrically or pneumatically depending on whether a chemical generator or gas bottle system is used. It seems reasonable that hitting the panel with enough force would break the latch or otherwise cause it to open.

The scenario where ALL the masks drop, which the article somewhat implies but doesn't outright say, sounds fanciful to me.  A cork striking an individual mask unit might cause that individual unit to drop its masks (so if the unit was for one side of a 3/3 seating arrangement, the 4 masks it contains would all drop), but the masks in Boeing airplanes at least are electrically activated, so physical damage to the cabin ceiling seems unlikely to cause a mass deployment.
Now, if the cork happened to hit the switch in the COCKPIT, then perhaps all the masks could deploy then.  Normally the masks drop only when the system senses a cabin altitude in the range of 15,000' MSL, but there is a switch the pilot can use to drop the masks if you get the rapid depressurization up high and the automatic system failed.  Typically this switch is safety wired shut for obvious reasons, but if some operator didn't use safety wire and the cork hit the switch JUST RIGHT, then this scenario might work.  (We're talking a one-in-a-million shot here, but it might be possible.)
There are stories of hard landing sometimes dropping one or a few rows of masks -- this happens when the latch on the container isn't quite as secure as it is supposed to be, and as long as nobody reaches up & activates the flow of O2, it's a pretty simple maintenance procedure to repack and restow the masks.  (If somebody DOES pull down & activate the O2, then the oxygen generator canister for that unit has to be replaced, and that's a more lengthy process -- especially if the generator isn't on hand at that airport!)  So a physical shock from a flying champagne cork being enough to drop the masks from ONE unit isn't too far fetched, but unless you're talking about a really old design where the signal to all units went out mechanically instead of electrically, I have a hard time believing this scenario. (Please note, I'm not claiming that even in the oldest designs a mechanical linkage connecting all the units would necessarily be present -- just that modern designs are NOT mechanical until you get to the latch on an individual unit.)
And noting that the plane isn't allowed to fly with masks hanging sounds right... but 7 hours just for repacking one unit seems rather lengthy.  I'd suspect that the paper took the facts of ONE unit deploying, and artfully crafted it to sound like lots or all of the masks dropped.  Sensational writing?

Yep, sounds fanciful to me too.  I think the chances of a cork making it to the cockpit with enough force to even disturb the paperwork are not even worth considering.  I suspect that you are right.  I can't see how this could have happened.  Other sources quote 1.5 hrs delay which would accord with one unit. Sensational writing?  In the media, concerning aviation?  Whatever next? ;)

Even the article concludes with "In line with safety procedures the captain took the correct decision to divert so that the cabin crew oxygen masks could be reset.", clearly indicating that it was not all masks, only those above F/A seats near the galley, so one or at most two units.

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Do airline pilots routinely carry out "gross error" checks?
Malaysian MH132 seems to have had a bit of a mix up over flight plans.  It appears to be a non-event but it does make me wonder if "gross error checks" are commonly used by airline pilots.
I'm only a PPL(H), but gross error checks are built in to me from training. For example, "after setting off on the first leg, the compass should be pointing NE which means the town should be on my right and the river on my left".
Some commentators have remarked that the captain did a "great job" in recognising the unexpected heading and querying it.  To me, it seems like basic airmanship to recognise and query it and leads me to also wonder if the crew studied the route during pre-flight and after it was loaded into the FMC.
Can any airline pilots add facts to the story?

Here's a brief and less  dramatic description of what happened. I can't answer your general question about airline procedures but it looks like in this specific case the pilots were indeed checking their course and were aware of the error.

@Pondlife Thanks for that, I hadn't seen the two routes.  So not even a non-event, one big yawn.  Why did this even make the media? (OK, so I'm smart enough to know the answer to that one).  Still interested in hearing whether gross error checks are part of SOPs.  Cheers

I agree, Malaysian MH132 was a Non-Event.   If the pilots had any concerns over the slight variation in their routing, they should have mentioned it before they loaded it into the Flight Management System (FMS).
I currently fly long haul B777 for a major airline. I can't speak for others, but at the airline I work for we do many "gross error checks". Some are done in flight planning, some during the loading of the FMC, and some are done during the flight.
While still in the flight planing area, the Captain and First Officer will look at the total routing in regards to wind, temperature, turbulence, volcanic activity, etc.  Often a call is made to the dispatcher who generated the Operational Flight Plan (OFP) to discuss the routing, expected levels of turbulence, and fuel reserves.  About 20% of the time the Captain will request a slight change to the routing or additional fuel be added.  A new OFP is then generated and it will be designated "Release 2".
Both the flight plan route and ICAO flight plan route are printed on the OFP and we compare the two.  I have never found an error in the 17 years I have been doing it, but we still do it every time.
Once the Captain, FO and augment pilots ALL agree, the OFP is officially accepted and we head to the aircraft. (up to 4 pilots on Ultra Long Haul flights)
When we get to the aircraft we run though the basic preflight checks, and then the flight plan is downloaded to the FMS using the Aircraft Communications Addressing and Reporting System (ACARS) digital datalink system. The departure runway and SID is also loaded at this time.
Before we "activate" and "execute" the routing, the Pilot Monitoring (PM) reads the entire FMC routing and the Pilot Flying (PF) compares it to the OFP.
After runway takeoff performance data is loaded we then check the "Legs" page of the FMC for a logical sequence of legs. We generally just check to make sure there are no course reversals on the various legs of the route.  If there are any user defined waypoints (Lat/Long waypoints) we perform bearing and distance checks of  the FMC against the OFP. This is done with the the Heading Reference switched to TRUE heading instead of Magnetic for more accurate numbers.  These bearing and distance checks are not done for "Named" waypoints, only Lat/Long waypoints.
Wind data is then requested and then loaded via ACARS.
Next is a check of the FMC distance compared to both the OFP distance and Great Circle Route distance.  The FMC distance is almost always somewhere between the OFP distance and GCR distance. This check is one sure way to spot a routing entry error.
Finally a check of the FMC and OFP predicted fuel over destination is done. If there is a large difference, it means either the routing or winds were incorrectly entered.
Once airborne there are more checks:
In the old days before GPS we would check the accuracy of the Inertial Navigation System (INS) as we approached the coast for any over water flight. This was usually done using a VOR/DME fix.
Once airborne we again check bearing and distance as we approach each user defined waypoint (Lat/Long waypoint). Once past the waypoint by about 5 mins we then plot our current Lat/Long on a plotting chart to insure we are still on course.

Thanks Mike.  I'm grateful for your detailed answer.  In particular, I wasn't aware that ACARS can be used to load the plan.  Don't know why, but I pictured one pilot keying it in with the other watching what's going on!  So last century I guess.  Thanks again.

Manually entering the route isn't really last century.  We only started using ACARS for the route about 10 years ago. Sometimes the system doesn't work and we have to enter it manually. Some airlines,  or some aircraft types, do not use ACARS for data entry, they may only use it for communication.

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Why select the transponder to TA only in the case of engine failure?
Part of the 777 engine failure checklist is to set the transponder to TA only.

What is the purpose of this step?

@mins, Ah, thanks.  Makes sense.  So is the design such that an aircraft receiving an RA from one only transmitting TA will take more "drastic" action?

The intruder will be resolving in "uncoordinated RA" mode, the vertical rate should be increased as soon as the separating distance crosses thresholds (reference is the paying  RTCA DO-185 document). From this document: "Other TCAS  II aircraft can generate (uncoordinated) RAs. A “TA-only” aircraft will be treated as unequipped by other TCAS II aircraft. This  will  allow  them  full  freedom  to choose the most effective RA."

The TA mode is selected when there is an issue, basically telling the 'other' aircraft that you cannot maneuver if a RA situation occurs. The other aircraft has to do all the maneuvering (as they can still generate RAs).
From Eurocontrol ACAS II bulletin from Dec 2012:

In the TA-only mode,the equipment still performs the surveillance function (i.e.  it  scans  for  proximate traffic) but it will only generate TAs. Other TCAS II aircraft can still generate uncoordinated  RAs  against  the  aircraft which has its TCAS II in TA-only mode.
The TA-only mode may be explicitly specified in operating procedures as a response to in-flight malfunctions such as an engine failure with consequent aircraft performance implications or a pressurisation system failure followed by an emergency descent.

I would think the uncoordinated response will occur sooner to provide more separation, is that true?

Where "you cannot manoeuvre" mainly means you can't achieve the rate of climb normally expected when "climb" instruction is given.


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Communication between airplanes
Hypothetically, can 2 airplanes (or rather 2 pilots) agree on a frequency that they know it's not used by any airport or service in the area and use it to communicate between each other, for whatever reason? Would it be legal to do so?

I don't know about other countries, but in the US, it's both possible and legal, as long as the frequency you pick happens to be one of the ones authorized for air-to-air communications. This AOPA article has more info, or you can check the official list at 47 CFR 87.187, but the basics are:

122.75 for regular fixed-wing aircraft.
123.025 for helicopters.
123.3 or 123.5 for gliders and balloons.
123.45 (a.k.a. "Fingers") was originally for flight testing, but has become a de facto air-to-air frequency just because it's so easy to remember. (So, technically not legal, but that doesn't stop anyone.)


text:
How is airliner pressurization being coped with at high altitude airports?
Cabin altitude in a passenger airliner must not become more than 8,000 ft, but what if an airliner flies to or from an airport at a higher elevation, such as Quito and La Paz? How is onboard pressure regulated when more pressure is required onboard than there is outside?
Most likely the cabin altitude is adapted eventually, but at what point of flight?


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Why do some planes drift to the left in flight simulators?
In different flight sims I've experienced the same effect on certain planes: when accelerating on the runway, the plane drifts to the left as if the rudder was set to yaw left. You can counter this by actually yawing right to somehow remain on the runway. When you're in the air the plane is rolling to the left as if the ailerons were set to roll left, and you have to counter this by actually rolling right. If you kill rotation, the plane would start to roll left again.
An example anyone can see is the SR-22 Cirrus in Google Earth's flight simulator (the F-16 doesn't do that). But interestingly I've seen the very same effect in other ones. What's the reason behind that strange behavior? Ailerons and rudders are set to 0 (no roll/yaw), so they obviously aren't or shouldn't be causing that.

let me guess, it is only with single engine prop aircraft

The simulator behaves just like the real thing.
Propeller powered aircraft with a prop that rotates clockwise will drift left during the takeoff roll if not corrected.  This is caused by a left turning tendency due to four distinct effects: P-factor, torque, spiral slipstream and gyroscopic precession. Of these effects, torque and spiral slipstream cause the most left turning tendency during a takeoff roll.
Engine torque creates additional loads on the left main landing gear which, in turn, increases the force of friction on the left main wheel(s).    Consequently, this creates a yawing moment to the left during a high speed taxi or takeoff ground roll.
Spiral slipstream or ‘corkscrewing effect’ is a result of propwash rotating in a clockwise spiral along the airplane’s long axis.  This impinges on the left side of the vertical fin and tail boom, creating a yawing moment to the left as well.
This combined yawing moment should be counteracted with right nosewheel steering and/or differential braking and right rudder as the speeds increase and aerodynamic control surfaces become more effective.
On tailwheel airplanes doing three point takeoffs, P-Factor will enter in to the left turning tendency, but only as speed increases and a nose high attitude maintained.  P-Factor’s effect will largely depend on the airspeed and angle between the propeller shaft and the relative wind.
If the airplane is equipped with an engine and propeller that rotates counterclockwise, these effects are reversed and a right turning tendency happens during the takeoff roll.  One famous example were the Supermarine Spitfire and Seafire variants powered by the Rolls Royce Griffon engines, which could be temperamental during the takeoff roll for this reason.
Twin engine airplanes equipped with counter rotating propellers and single engine airplanes equipped with contra rotating propellers can largely negate much of this left turning tendency as such systems counteract the torque produced on the airframe, eliminate P-Factor and gyroscopic precession effects, and reduce the spiral slipstream effect to a greater degree.  The downside is that these systems increase the weight, complexity, and costs of the propeller and power plants used, and this generally deters their integration into light airplanes by the manufacturers.

Thank you. So pilots actually yaw to the right on the runway to correct that. Pretty dangerous. But shouldn't planes whose propellers rotate counterclockwise then drift right?

You will apply right rudder during the takeoff roll to counteract the effect of torque and track the A/C on centerline

And yes airplanes which have a propeller which rotates counterclockwise will drift to the right during the takeoff role. See a Rolls Royce powered spitfire as an example

See my comments above about counter rotating and contra rotating propellers.

@Betternottell it is not that much problem on take-off as the pilot has enough time to apply the correction, but it may be dangerous during go-around – crashes of small planes during go around because the pilot fails to apply the control correction when adding power, letting the plane pitch up and roll over to the left, are unfortunately somewhat common.

The only time precession is really noticeable is when you raise the tail early in the takeoff run of a taildragger.  If you force the tail up sharply, early, you will get quite a hard swing left from precession, depending on the weight of the propeller relative to the airframe.

@JohnK In MFS X the P51 Mustang is swinging to the left even in level flight up in the air, unless corrected by aileron. But the P51 is a race plane in it, so perhaps it is even intended that it flies to the left in a circle.

No airplanes don't normally turn by yawing;  it's mostly P factor and torque from the high power & mass of the propeller relative to the airframe.  P factor offsets the thrust line to the right any time the propeller isn't perpendicular to the airflow, which is most of the time, to some extent. You normally apply rudder trim as required to stop the pull.  On takeoff the Mustang requires a specific rudder trim setting as part of its checklist.  Torque and yaw reactions to engine power complicated aerial gunnery, since every time you moved the throttle the nose would move and throw your aim off.

@Better not tell.  No, that’s just symptomatic of a poorly trimmed airplane.  Larger more powerful airplanes like that will require trimming after every power change in order to neutralize control pressures in all three axes.

@StephenS The thing is if you set the rudder and/or aileron too right the plane will drift right. There must be some proper rudder setting.

@Betternottell You don’t “set” the rudder (except maybe in cruise) because the amount needed will vary as engine power and airspeed change. This is the origin of “flying by the seat of your pants”, actually: you will feel a sideways force in the seat if the plane is yawing, and part of primary training is to create the reflex to move your feet on the rudders to make that feeling go away. Obviously, you don’t get that feeling at all in a simulator, so you have to consciously center the ball on the inclinometer.

@Betternottell In your first comment on this post, you say "pretty dangerous." Why?


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How did the unpressurized Cessna Grand Caravan fly at 43,000 ft?
Recently I've seen a Cessna 208 Grand Caravan flying as high as 43,000 ft MSL on flightradar24. Perhaps it's still in the air if you look over eastern Canada (the Grand Caravan has a unique shape from the other planes on fr24, looking like a sailplane). The Grand Caravan is unpressurized, does that mean all passengers wore oxygen masks? Or was it just fr24's error to portray a Grand Caravan that high (and if so, how would such error occur)?

I wouldn't want to fly to FL430 on oxygen.

You'll likely get the bends after 30 minutes at that altitude.  You don't really want to spend significant time above 30k unpressurized.

@ZeissIkon I think it would be a fascinating experience.

Linking to the FR24 track would be very useful in this case. Write down the URL on a piece of paper if you have to, if you can't copy/paste...

Voting to close as "needs more details" until you can provide a flight-tracking link or other identifying information. In particular, if you have the registration number of the aircraft we may be able to look it up and see that it used to be assigned to a C208 but is now (e.g.) a Lear 75. In any case, asking a question about a track observed on FR24 and failing to identify it any more than "Over Eastern Canada an hour or so ago" is superbly unhelpful.

@randomhead I also mentioned that the Caravan looks like a sailplane on fr24 which makes it look different than the bizjets and airliners. You can set a filter to show planes around FL430 only, there the (alleged) Caravan is unique and easy to find over eastern Canada. I cannot post a URL because I'm using fr24 as desktop version on another medium.

Related: https://aviation.stackexchange.com/questions/46736/how-can-a-helicopter-show-up-at-29000ft-547kts-at-flightradar24

Flight Radar 24 works mostly by receiving and decoding ADS-B (Automatic Dependent Surveillance - Broadcast) messages from aircraft. In these messages, there is no data on the aircraft type.
To display the information about the aircraft, Flight Radar 24 has to use the 24-bit address (a code similar to a MAC address in networking) that is uniquely assigned to an aircraft, and look up the aircraft information in a table in their database.
What happened here, is likely one of the following:

the aircraft was flying at 43 000 ft, but was transmitting the 24-bit address of another aircraft (this can happen due to various reasons, from a mechanic putting in the wrong 24-bit address, to electric connection failures). The lookup therefore picked the wrong aircraft (a Cessna Caravan) from the table.

the aircraft was flying at 43 000 ft, and was transmitting the correct 24-bit address, but the table was wrong. This can happen, for example, when a new aircraft is registered on an address that previously belonged to another aircraft (a Cessna Caravan in this case), and the table used by FR24 has not yet been updated.

the aircraft was the Cessna Caravan, but the altitude encoder was erroneously encoding 43000 ft instead of the actual altitude. This occasionally happens, but if the aircraft is receiving Air Traffic Service and is within (secondary) radar coverage the discrepancy would be noticed by ATC very quickly. Depending on the airspace class and the controller workload, the flight may either be allowed to continue based on verbal altitude reporting or the pilot would be asked to land or otherwise leave the airspace.

Without more information about the date, time, location and/or aircraft registration of your observation, it is impossible to discern which of the above cases applied here.

Would the pilot be asked to land? I thought the controller would ask the pilot to report altitude and proceed with that.

@JanHudec It depends a bit on the airspace class and the workload of the controller. If the controller can accommodate the flight based on altitude reporting, then that is an acceptable, albeit temporary, work-around. In other cases, the pilot will be asked to leave the airspace. The pilot may choose to land or proceed to move to Class G airspace or another airspace where no radar services are required for the flight. In any case these kind of errors do not persist for long.

In the US I would think it exceedingly unlikely that an aircraft would be asked to land or exit certain airspace due to a Mode C failure. There is nothing in the .65 that directly requires that; instead as @Jan said there is a procedure and phraseology to use when the Mode C is off by 300 feet or more (namely, ensure the pilot is using the proper altimeter setting and then trust their reported altitude rather than Mode C). I have even seen a target with Mode C failure permitted to continue flying in Class A airspace.

Why does such a good answer have so few upvotes?

@Betternottell that has to do with how this site works. Most of the visits to a question are within the first 24 hours after it was posted. And, as soon as a question has an accepted answer, less people are visiting and voting. My answer came more than 24 hrs after you asked the question. By then, most frequent visitors had seen the question and the accepted answer and so they don't bother to check again. If you want to attract good quality answers, you have to write a good quality question, but also be patient. Don't click the accept mark within 24, perhaps even 48 hours.

But although it is nice to receive up votes, I don't care that much. If I have the time to write an answer on a topic I am familiar with, I am just happy to be able to share the knowledge and help out. I see you are new here: welcome to the site.

Thank you but I ain't new, I just better don't tell who I am. On a certain SE site (albeit not this one) they were pretty mean to me. Your answer is qualitatively the best and should have the most upvotes. I think people were visiting this question for multiple days because of comment debates. If any more people really don't bother to upvote (and I myself cannot upvote at all, I'm unregistered), the accepted answer should be pinned first again.

The Wikipedia page for the Caravan family lists the service ceiling at 25,000 feet.  I think it's very unlikely someone got one (any version) as high as 43,000, and less likely yet on oxygen in an unpressurized cabin.
That means this is probably an error in altitude reporting.  How such an error could occur is difficult to say -- I'm not familiar with fr24, so don't know how they obtain altitude data.
Note also that "At 40,000 feet, 100-percent unpressurized oxygen provides the equivalent of ambient air at 10,000 feet."  It would be unnecessarily risky to fly higher than this without some kind of pressure support, and would require full mask oxygen, not the cannula often used by private pilots who venture about 12,000.

Or the wrong transponder code...

I worked at a busy enroute facility. I would get calls asking that we relay a message to a pilot and the caller would provide the location and altitude. Sometimes, the aircraft would not even be in our facility's airspace, much less the location/altitude given. The online tracking tools are generally fairly good, but do have some problems now and then. A couple of times the caller would argue, and I would point out that I was using a certified ATC system, and they were not.

It might have been a wrong transponder code indeed, perhaps.

It's also possible that the pilots thought "If our passengers are forced to wear FFP2-masks, why not just let them wear oxygen masks and take them to altitudes they likely never were before in their lives". Service ceiling is not the same as absolute ceiling, after all.

"Service ceiling is not the same as absolute ceiling, after all."  While true, they seldom differ by this kind of margin.  Usually you can trick, cajole, or coerce an aircraft a few thousand feet above service ceiling -- unless the service ceiling is a limitation of the onboard oxygen system, for instance, which would mean the airplane can fly higher, but crew and passengers cannot.  I'm pretty sure oxygen is good above FL300, but it seems to be limited to about FL400 without pressure support.

That's an interesting link. The author often voices his own opinion but I can understand this as he lost a friend to hypoxia. May he rest in peace.

Here’s a ForeFlight screen shot of the local flying club’s Cessna 172 pretending it’s a U2 87,500'. So I’m guessing bad data.

Or maybe it's a U-2 pretending to be a Cessna of a local flying club


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What is that plane flying over Missouri at 59,700 ft?
On flightradar24 I saw a plane flying as high as 59,700 ft at a bit more than 40 kn (pretty slow to fly at such altitude) over north-east Missouri at 1:18 am CST. Fr24 didn't tell what plane that was and I guess it's an experimental plane or drone. It could also have been an espionage balloon, but it was portrayed as a winged aircraft. Does anyone know or have an idea what that could be?

Any conventional aircraft capable of flying as slow as 40kn is not going to be capable of reaching 60,000ft. Probably, you saw a weather balloon, assuming that it wasn't just an artifact of FR24's systems. There are many questions here about such artifacts.

@CatchAsCatchCan But it didn't have to be a conventional one. I once saw a U2 on fr24. The experimental NASA Pathfinder and Helios can/could reportedly reach higher altitudes at slower speeds. https://en.wikipedia.org/wiki/NASA_Pathfinder Weather balloons are usually portrayed as balloons on fr24, but this one was portrayed as an airplane so if anything it might be an espionage balloon I guess.

@CatchAsCatchCan It couldn't have been an artifact, for it had a route (it flew from the north) showing where it launched from and the aircraft's values changed with time.

A screenshot and link to FR24 would be great.

@Bianfable I'm using fr on another medium where there aren't the cookies to register into this account so I wouldn't have the SE account on the other medium. But anyone can trace it back since fr allows for that. Just set the filter to planes above 59,000 ft for instance and enter the time (01/29 1:18 CST which is 7:18 GMT).

@Betternottell U2 stall speed at 50,000ft is around 105kt, indicated. True air speed would be much higher. See this question

@CatchAsCatchCan That's interesting, but I wasn't implying the U2 flew at 40kn. I meant the NASA Pathfinder and Helios and was referring to the fact that fr24 doesn't show civilian planes only.

A high-altitude balloon, identifier HBAL577, registration N257TH.

ADS-B Exchange track
FR24 track
FR24 replay
(FR24 may be storing "altitude" as an unsigned 16-bit integer; notice how the altitude seems to wrap around back to zero in the vicinity of 65,500 feet.)
More information available at this FR24 blog post. Notice how they say "Because the balloons do not necessarily transmit their type, some may appear as aircraft icons until we are able to update our database."
The FAA's N-number Inquiry site shows that "N257TH" has been reserved by Raven Aerostar, who formerly collaborated with Google on Project Loon (although it is not actually shown as assigned to any aircraft, which is why FR24 did not know to show it with a balloon icon). This article from The Drive notes that Project Loon was shut down in January 2021 but Raven Aerostar continues to develop its balloon technology for military applications.

You know you're flying high when your flight causes integer overflow.

They still use 16 bit ints in 2022?

@void_ptr kudos for choosing that username for posting this comment

@KevinKostlan, smaller ints are used more often in practice than you'd think. It makes sense for fr24: there are about 10,000 planes in the sky at any given moment, and fr24 logs every 5-ish seconds. If speed, direction, and altitude all are stored in 16 bits instead of 64-bit doubles, that saves about 1 GB of data storage per day, which adds up when you've got a few years of data, plus backups, indexes, bandwidth considerations, etc.

@Jason I'd expect any long term data storage to losslessly compress data - greatly reducing data needs.  Using 16, 32 or 64 bit raw data makes little difference then in terms of compressed data size.

@chux-ReinstateMonica I would assume the goal is more to reduce bandwidth on the receiving station/server link.

Personally I'm surprised they're storing the value with single-foot precision. I'm not sure exactly how the altimeter reports altitude but ATC only sees 100-foot increments... it seems storing the altitude in units of 25' and then using JavaScript or whatever to multiply that by a factor of 25 for display would be sufficient. But then again they probably don't track a lot of flights above FL655.

@randomhead ADS-B transmits barometric altitude in increments of 25 feet up to roughly FL500 and then in increments of 100 ft. Alternatively, it can transmit GNSS altitude in metres, so converting that to feet will give all kinds of weird numbers.

For all this talk about optimizing how to store altitude numbers, it is probably just laziness. No one wants to patch, debug, and retest the ancient (spaghetti) code just to fix a few balloons.


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Why do B787 Dreamliners more often fly at 42,975 ft rather than at 43,000, according to flightradar24?
The A350 and the B787 are the only passenger airliners that can be seen cruising as high as 43,000 ft on flightradar24 (with the exception of a B747 I once saw). However, the B787 is more often found cruising at 42,975 ft rather than at 43,000, unlike the A350 which is always seen cruising at exactly 43,000 ft. Sometimes the B787 flies at 43,000 ft but more often at 42,975 according to fr24. If so, why? If not, if it is actually cruising at 43,000 ft, does its portrayal on fr24 have to do with its transponder's position or something?

I suppose it's an artifact of how the mode-C encoder rounds altitude (the difference is by one as the precision of that message is 25 ft) and/or how the altitude controller is tuned.

Tail fin might be a little tall

@JanHudec But A350s are always portrayed at 43,000 ft so still they must fly somewhat higher. The blue planes are often portrayed at altitudes like 42,996 ft or 43,004 ft or the like, btw.

Despite what FR24 and FA tell you, aircraft do not cruise at 43,000 feet except accidentally. Rather they cruise at FL430.  See this question for more information (I had a lengthy answer typed up here but then I realized it wasn't really relevant). I will second @Jan's idea that it is a simple rounding or encoding discrepancy, combined with observation bias on your part.

@randomhead I know it's the flight level but A350s never cruise at 42,975 ft in flightradar. Nor do A350 or B787 ever cruise at 43,025 ft. Fr24 is actually showing the calibrated altitude and it's too much of a coincidence to consider the B787 mostly flying at 42,975 ft an encoding discrepancy while an A350 always cruises at 43,000 according to fr24. The "observation bias" just means you can filter in fr24 which planes you want to have shown, e.g. planes flying around FL430. Did you get it? If it was a matter of rounding, A350s would show the same discrepancies.

At the moment F24 sees 5 B787 above 42500. 3 of them showed 43000ft, 2 showed 42975ft.

"aircraft do not cruise at 43,000 feet except accidentally"  If you were cruising at FL430 on a standard day you would be at 43,000 feet MSL, and it wouldn't be an accident!  ;)

@DeltaLima You see? And if you look for A350s you don't find one at 42,975 ft.

There's something here I do not understand: are you saying that flightradar 24 measures and reports altitude to an accuracy of 25 feet +/- nothing???

@nielsnielsen I don't know exactly how fr24 measures the altitude, and blue planes are measured differently than yellow ones. It's the "calibrated altitude" so I'd say the yellow planes show the altitude from the plane's altimeter. As stated above, the blue planes even show values like "43,004 ft". But my question just asks why fr24 shows B787s often at 42,975 ft and A350s always at exactly 43,000 ft, regardless of whether these values are correct or not.

@nielsnielsen, it's the 1090ES ADS-B transport that transmits altitude to precision of 25 feet (accuracy of the altimeter is probably a bit worse still). It surprises me a bit that the satellites receive different messages (UAT?) that apparently have precision of 4 feet.

@Betternottell, all planes show the pressure altitude from the plane's altimeter, which is the only thing that makes sense. The blue means the planes are tracked by satellites rather than ground stations, and it looks like the satellites are receiving different kind of messages with different precision (UAT instead of 1090ES I suppose). But both are sent by the same instrument on board the aircraft.

@JanHudec So there's still no explanation why B787s are sometimes shown at 42,975 ft, unlike A350s. Perhaps A350's altimeters are less precise or follow a different atmospheric model? But when flying at FL430, each plane's altimeter set to 29.92 should show exactly 43,000 ft.

@Betternottell, My best guess is that the altitude PID or PD controller in the autopilot it tuned so it does not overshoot, so the altimeter ends up reading, say, 42,998 ft when 43,000 is selected (well, the altimeter is not accurate to 1 ft anyway, probably not even to the 25 ft at this altitude), and the encoder then rounds it down. But that's just a guess given some superficial knowledge about the operating principles.

@JanHudec, thanks for the information! -NN

This is a guess based on superficial understanding of the systems involved, but I'll make it an answer as it seems to be all that's available unless an engineer who actually worked on the 787 autopilot chimes in, because it's details I don't expect to be documented anywhere public.

So: there are two systems involved in this, the autopilot and the ADS-B encoder.
The autopilot is a feedback-loop controller. Probably a variant of a PID controller or maybe two chained controllers, the altitude one commanding a either pitch or vertical speed one. These controllers take the altitude, vertical speed and pitch and calculate some formula that determines whether to wind the elevator up or down. The coefficients are estimated from design and then fine-tuned in test flight.
Depending on how exactly the tuning worked out, the aircraft can settle at altitude few feet below or above the target – either the vertical speed is reduced fast and the airplane converges to the target altitude from below, but remains a few feet lower for long, or the vertical speed is reduced a bit more slowly and the plane overshoots a couple of feet and very, very slowly comes back down – the plane is a bit over 50 ft high, so a couple of feet off is more then good enough either way.
The other part is the ADS-B¹. The 1090 ES message (which is extension of the mode S transponder) encodes altitude with precision of 25 ft only. So it has to round the value it receives from the altimeter. And if it rounds it down, a couple of feet below might end up being rounded to 25 below.
None of this matters in practice. The controller only sees the altitude rounded to 100 ft. Anything else would be just distracting. The planes are tens of feet high, and the accuracy is tens of feet too. As long as the system can keep the plane ±100 ft of the assigned altitude it is good enough.
Apparently the satellites listen for different kind of messages (the UAT, I suppose) with different precision, so the altitudes are rounded differently for the planes shown in blue.

¹ FlightRadar 24 relies purely on information broadcast by the aircraft themselves, but for altitude so does the ATC. Barometric altitude has the advantage that barometer accurate enough to distinguish tens of feet is a simple and reliable device and was already available in early 20th century. But it measures pressure, not altitude, and due to weather, the geometric altitude can be quite different. That does not matter for separation, since there can't be any abrupt changes in pressure (air being fluid), so we know aircraft in the same area are at the same altitude if and only if they are at the same isobar. But it means we need to know the pressure and the aircraft barometric altimeter is the only instrument that knows that. So ATC also looks at the altimeter reading the aircraft transmits with the transponder mode C or ADS-B.

"there can't be any abrupt changes in pressure (air being fluid)," I'm pretty sure that extreme weather can cause some pretty extreme pressure gradients, e.g. tornadoes, downbursts, etc.

If the plane is in a tornado it has more to worry about than the altimeter being off by a few feet.

So A350s are obviously more easily to be put at the exact flight level. I guess that's because Airbusses more rely on their computers while Boeings require the pilots to navigate more on their own.

@Betternottell a) it does not have to be exact; maybe the A350 ends up overshooting or maybe it just rounds the value differently in the transponder. b) Both 787 and A350 are fly-by-wire to the same extent, the only difference is that A350 auto-trims while on a 787 the pilots still control the trim explicitly in manual flight – but that does not matter because autopilot does it in both cases anyway.


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Do airline pilots really put on an oxygen mask when alone in the cockpit above 25,000 ft?
The FAA requires a pilot to put on the oxygen mask if the other pilot leaves cockpit when cruising above FL250. It also requires that one pilot always wears an oxygen mask when flying above FL410. Do airline pilots really do that and if so, do they do it only over American soil and then put off the mask, e.g. when flying over the ocean? How would it be controlled, other than by sounds heard by ATC? And do bizjet pilots have these requirements as well or do these count for the large airliners only?

91.211  talks about a "quick donning" mask being available at FL350, required to be worn at FL410 or if solo on the controls at FL350. Where is FL250's mask requirement?

Yes, the requirement is that you can't fly above FL250 without having 10 minutes of oxygen available for everyone on board.  Pilots don't need to mask up until FL350, if alone, and only beyond FL410 with more than one pilot present.

I am a retired Canadian B767/B777 airline pilot and have flown over USA airspace many times. I have never put on an O2 mask while flying alone in the flight deck over FL250. None of my fellow crew members did either. Occasionally we might talk about the requirement, but we always justified our non-compliance because we had quick donning masks that were easy to use.
I have only flown at FL410 a few times and never flown higher than FL410.
I would be curious to hear from American pilots to see how faithfully these rules were followed.
See also:  Why are rules regarding the use of oxygen masks by commercial pilots often ignored?

Comments are not for extended discussion; this conversation has been moved to chat.

Pilot flight crewmembers would be required to follow FAA regulations when operating an aircraft in sovereign airspace over the USA including US territorial waters.  After that, you’re over international waters and required to comply with the regulations set by the international civil aeronautics organization (ICAO).  These do not conflict with US law, though they do not require you or another pilot flight crewmember to use supplemental O₂ at or above FL350.
That being said, while you may not be legally required to use supplement a oxygen above FL350, it is a very wise precaution to do so.  At FL400, Time of Useful Consciousness (TUC) is only 5-10 seconds.  In the event of a rapid decompression or similar emergency, the trauma or other effects of a serious catastrophe could quickly eat up all of your time of useful consciousness before you could get a mask on your face and start breathing oxygen again, posing a very serious risk to life and limb.  I would not be surprised if all the major international carrier SOPs require their pilot flight crewmembers to be using supplemental oxygen at all times while at their crew stations above FL400.

Comments are not for extended discussion; this conversation has been moved to chat.

As far as I know, there's no such thing as "regulations set by ICAO." ICAO has its "Standards and Recommended Procedures," but these have no force of law anywhere unless actually enacted in regulations from the relevant local civil aviation authorities. Also, while FAA jurisdiction of non-U.S.-flagged aircraft does end at the end of U.S. airspace, its jurisdiction over U.S.-flagged aircraft is worldwide. Generally, aircraft must follow (at least most of) their flag country's regulations regardless of where they are in the world at any given time.

I can attest to seeing Fedex pilots do this back in the 1990's. The first time I saw them do it I was concerned and asked the FE if something was wrong and he told me about the regulation and that it was normal procedure.


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Twin-Engine WWII Japanese Bomber
I recently inherited a number of pictures taken by my father in August & September 1945.  During the process of Japanese surrender at the end of WWII.
Several are of captured aircraft, most of which are identifiable, but I can't find information about this one...

The picture was taken in Borneo.  The aircraft has the white cross painted over the rising sun emblem, which identifies it as captured spoils of war.
The photo has a handwritten note on the back - simply "Captured jap bomber".
Is is a bomber?  It seems to have windows in the fuselage.

Google Lens suggests that this is a photo of a Tachikawa Ki-54 (trainer / transport). And I think Google Lens is right.
https://en.wikipedia.org/wiki/Tachikawa_Ki-54
Tachikawa Ki-54 photo source

The landing gear looks more like a Ki-54 than a Ki-57.

@qq  jkztd I believe you are right and I edited the answer to include that photo and link.

Looking closely at the camo details, the a/c in photo in answer is clearly the exact same individual airplane as the one in question--

@quietflyer Judging by the group standing behind the fuselage, just ahead of the cross, and the other group standing a bit in front of the plane, it may very well be two photos taken at nearly the same time, of the same plane in the same place with the same people around. Hard to say with how blurry the Q photo is.

This looks remarkably like the very same plane.  The man on the right in your picture appears to be wearing an AIF uniform, those in my picture definitely are.  The ground and the background vehicles look remarkably similar.  A great find.

I'd also point out that there's a mark on the right front tire, near the bottom, in both pictures, in the same location, suggesting that the plane has likely not moved at all between one picture and the next, unless it's just coincidence the wheel wound up in the exact same rotation...

Is it significant that the starboard prop in both photos is stopped in the horizontal position? Or is that a feature of the engine?

Possibly a Mitsubishi_Ki-57?
It's a radial twin and looks to be a similar size and shape, with a row of square windows at the right height and spacing.

Image courtesy of commons.wikimedia.org
The Ki-57 was a passenger transport derived from the Ki-21 bomber but the 57 looks to be missing the nose glazing and the dorsal glass house of the Ki-21

The Ki-57 was also my first guess, but the main landing gear is different.

The tailwheel placement is also very different.

Also too many panes in the cockpit side.


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Find Cl from Cp values
i have a code of the panel method where I can calculate all the Cp values.
my job is to calculate the Cl from these Cp values, and I understand that to do that I have to integrate the Cp over the airfoil.
on the Xfoil manual I find that the program to calculate the Cl integrate the Cp in dx, where x=xcos(alpha)+ysin(alpha) where alpha is the angle of attack.
i have the information but I don't know how to do that on Matlab, someone can help me? thanks!

it's similar but I don't really know how to implement this approach in Matlab. someone can help me? because I know theoretically how to do but I don't know how I can calculate the integral of cp on the airfoil. thanks!

Matlab support/training is outside the scope of this site. You can try on stackoverflow.com, but the way your question is worded right now they probably won't help you either. What did your research to solve the problem on your own bring up, what have you tried so far, where exactly are you stuck?


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Are there retractable, compact hang gliders?
I was wondering if there were hang gliders that you could, say wear on your back, in which the wings would retract into the suit. Kind of like the falcon in the mcu? If they aren't there, why not?

Is it possible that the answer is "a parachute?"

Not for humans.  The creatures that have something like that are called "birds" and "bats".

@quietflyer: Batman has also something similar . If I'm not wrong it works via piezoelectric effect

Collapsible hang gliders, in the sense I presume you mean here do not exist.
With retractable/collapsible hang gliders, the problem would be the size of the contraption. It would have to be very large to reach a reasonable glide ratio, so being able to fold down compactly is not really a viable concept. Also, the mechanism to enable it to fold down would be complex (because of the size), and thus quite likely heavy.
I'm guessing paraglider is not what you have in mind here, as it's deployment is quite an ordeal, not resembling the quick 'n easy "launch" of Falcon's wings in Marvel Cinematic Universe.
The closest match, although not a glider in any sense, would be Yves Rossy's winged Jet pack. It could be designed as collapsible, and it is somewhat transportable, not for longer distances though as it weight about 50kg. I'll venture a guess Rossy's jet pack has the glide ratio of approximately matching the space shuttle, which in turn was called the flying brick for more than one reason.


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At what altitude can the sonic boom no longer be heard at sea level?
As others have stated here, you can hear the sonic boom of planes flying by at a fairly high altitude, ~80 000 ft. At what altitude would the sonic boom no longer be noticable at sea level? To add to that, is there any relatively "simple" method of estimating sound dissipation with altitude?

Interesting question. We don't hear meteors at all, meteors fly through the mesosphere, above 160,000 ft (49 km). Therefore, the hearing limit is probably somewhere between that and 60,000 ft (18.3 km), Concorde's maximum cruise altitude.

The larger the object, the louder the sonic boom.  A supersonic bullet produces a "click" when it passes by (not that I recommend trying to experience this personally), and that click wouldn't be audible beyond a few hundred yards; the "click - bang" would become just the "bang" of the weapon firing.  Given this dynamic, there won't be a single altitude that "the sonic boom" doesn't reach the surface; the altitude would be affected by the size of the aircraft involved.

I'm gonna say a .308 makes more than just a click at supersonic speeds.

@Betternottell We don't hear meteors at all  ... I beg to differ. In 2013, windows were smashed, buildings were damaged and people were injured as a result of the shockwave of a meteor near Chelyabinsk, Russia. Smaller meteors may be heard too, but the smaller the meteor, the more faint the noise it makes. Note that due to distance the sound has to travel, there is a long delay between the sighting of the meteor and the arrival of the sound.

@DeltaLima I meant the usual meteors (falling stars) such as Perseides or Leonides, not exceptional meteoroids surviving the stratosphere.

“Noticeable at sea level” will be a function of multiple parameters in addition to altitude (e.g., Mach, flight profile, weight/lift, aircraft shape, day-of atmospheric properties, ambient noise at ground level, inside/outside, etc.). That said, relative trends of boom strength with altitude can be found multiple places in the NASA’s so-called boom bible, or SP-2014-622. For example, see page 135 and figure 5.3.

As an observer, you hear the Sound Pressure Level (SPL), which comes from a source emitting sound at a given Sound Power Level (SWL).
According to this source, the SWL of a supersonic plane may exceed 200 dB. In the same source, a formula for computing SPL a given distance is also shown.
You can find an online calculator for that here.

Please include the formula inline. The source link is a good citation, and appropriate for extensive supporting information, but the core piece of information should be directly included in your answer and not require following the link offsite.

Interesting beginning of an answer, but voting down for the reasons @MaxPower cited. There is no actual answer here, just a roadmap toward potentially calculating one.

The International Space Station orbits the Earth at Mach 25, at an average altitude of about 250 miles (400km) above sea level, and we don't hear any sonic booms from it. In space, the air density is so low that there aren't enough particles to bump into each other to create a boom.
The ISS's avg altitude is 250 miles, but it can get as low as 174 miles, and we don't hear any booms even then. I assume then that the altitude where the boom goes away is somewhere between 60,000 ft (where the Concorde flew and did for sure emit a boom) and 918,000 ft (174 miles, the lowest altitude of the ISS, which for sure doesn't emit a boom).
Actually, gonna add this as well: "space" technically begins at 50 miles or 264,000 ft (well, unless you're not American -- then it begins at 62 miles -- but I digress, the difference is so small it doesn't really matter for what we're discussing). Because there are already so few (if any) air particles up at those altitudes, I think it's safe to say that a boom is extremely unlikely or impossible even at 50 miles (or maybe even a little under 50 miles) above sea level.
Best bet would be to not use a plane, though. Planes need to take off on a runway, gradually climb, gradually descend, and land again on a runway. Even though it won't emit a boom in cruise, it will for sure emit them in ascent/descent, which will be pretty long distances, since you need a long distance to climb to / descend from your cruise altitude. Those distances will be not-insignificant percentages of your total flightpath.
So instead, launch straight upward like a rocket, then transition to level flight once you're close to your cruising altitude (take 50 mi as the transition altitude, and 62 mi as the cruise altitude, just for example), cruise, then transition back to vertical descent near your destination, and land vertically like a rocket. The sonic booms, which will only be emitted in ascent/descent, will be emitted in one small isolated ring surrounding your launchpad & one surrounding your landing pad. Which won't be a problem if the areas immediately surrounding your spaceports are pretty empty. Easy to have, since most big airports today are surrounded by empty space anyway (bc they are located outside their served cities). Or just put your spaceport in a lake/offshore.

Welcome to Aviation SE, T_S.  Can you edit your answer to stick to aviation, not spacecraft?  That would be on topic at other SE sites, not this one.

@CamilleGoudeseune but was there anything inherently incorrect about the answer? Just curious, please let me know if there is

Nothing incorrect.  Clever to contain the boom by flying vertically while in atmosphere.  But still off topic, so move most of these ideas to a spacecraft SE site.

@CamilleGoudeseune My answer wasn't off topic. I gave the relevant answer to OP's question. If anything, by your logic, OP's question would be off-topic for the Aviation site, and he should have asked it on the Space site. It isn't off-topic to give the most relevant answer to the question presented.